Please refer to FIG. 1. As shown in FIG. 1, although a light-blocking sheet is disposed in a projection system for blocking the stray light 10, the stray light 10 coming from all directions is still possible to enter the projection lens 12, and consequently the contrast of the images projected from a projection system is lowered.
Please refer to FIG. 2. A light-blocking sheet 14 is disposed in the projection lens 12 for preventing stray lights in the projection system from being projected through the projection lens onto a screen (not shown in the figure), and for preventing the contrast and the definition of the projected images from lowering. Because the light-blocking sheet 14 is fixed to the inside of the projection lens 12, when light is projected from a light valve 16 into the projection lens 12, the light-blocking sheet 14 disposed inside the projection lens 12 can block ineffective lights for making images projected from the projection system more clear and increasing the contrast of images.
Conventionally, the light-blocking sheet 14 is disposed inside the projection lens 12 and cannot be adjusted after the projection lens 12 is assembled. Thus, the appropriate position of the light-blocking sheet 14 is determined after a complex calculation. This is time-consuming and the cost is higher. In addition, the position of the light-blocking sheet 14 is not suitable for every different kind of optical system. When in production, the variation caused by each component in the projection system cannot be corrected effectively. The common variations that affect images projected from the projection system include variation in the light valve 16, variation in the assembly of the projection, variation in the disposition of the light source and variation in the production of the projection lens. Although the light-blocking sheet 14 is disposed inside the projection lens 12, the light-blocking sheet 14 is fixed to the inside of the projection lens 12 and cannot be adjusted according to the variations in other components of the projection system. Consequently, the quality of images projected from the projection system is not optimized. The contrast and the definition of images cannot be increased, either.